The present invention relates to a process for distilling water such as sea water.
Various distillation processes and apparatuses are known such as a multiple effect evaporator, a multistage flash evaporator and a solar distillation apparatus, and are availably employed for purifying impure water or desalting sea water.
A known distillation apparatus such as a multistage flash evaporator is complicated and large, and the heat efficiency is not so good. The evaporator has also the disadvantages that it is necessary to reduce pressure or to make vacuous, that a large mechanical energy is required for conveying upward and downward water to be treated or for passing the water through, for instance, spiral pipes, and that the maintenance cost comes expensive.
Solar distillation apparatuses hitherto used are classified roughly into a so-called basin type in which a heat collector and an evaporator are joined in a body and a so-called separate type in which a heat collector and an evaporator are separately installed. The basin type solar distillation apparatus is generally inferior in the heat efficiency, but the construction is simple. On the other hand, the separate type solar distillation apparatus is good in the heat efficiency, but the construction is complicated and the operation cost is expensive.
In general, a basin type solar distillation apparatus is composed of a basin filled with a waste water or sea water in the atmosphere and a transparent cover therefor. The basin is lined with a material such as butyl rubber which is water resistant and absorbs well solar radiation. A heat insulating material is also employed for decreased heat loss. A glass sheet and a weather-proofing clear plastic film are employed as the transparent cover, and the cover is installed with a slight gradient. The sunlight falls on the basin through the transparent cover and sea water so as to raise the temperature of sea water. When the temperature of sea water becomes higher than the surrounding atmospheric temperature by about 10.degree. to 15.degree. C., the evaporation becomes vigorous and the water vapor begins to circulate within the apparatus with air. The water vapor is condensed on the inner surface of the cover, and the water droplets run along the inner surface of the inclined cover to the side walls of the basin and is collected into a fresh water tank through a conduit. In such basin type solar distillation apparatus, the heat efficiency is less than 45% in summer when the highest efficiency is attained, and is about 30% on the average throughout the year. In order to improve the distillation efficiency, there have been proposed various basin type solar distillation methods and apparatuses, such as the use of a basin having a corrugated bottom to increase the evaporation area and the use of a water or brine cooling means for cooling a cover instead of air cooling, but they cannot raise the efficiency so high.